Method and system for reselecting an access point

ABSTRACT

A method for use in an access point includes receiving a request for a reselection message from a wireless transmit/receive unit (WTRU). The reselection message is generated in response to the request and includes: at least one first identifier of one or more candidate basic service sets (BSSs) that may be considered for reselection; a preference indicator associated with each candidate BSS, to indicate the BSS selection preference; an expiration time indicator associated with each candidate BSS, to indicate a time period that the BSS may be considered for reselection; an indication of a measurement to be taken on a BSS and used in a target BSS reselection determination; and a margin by which the measurement must exceed a corresponding value for a currently selected BSS. The reselection message is transmitted to the WTRU. A reselection response message is received from the WTRU, indicating the target BSS reselection determination.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/319,811, filed Dec. 28, 2005, which claims the benefit of U.S.Provisional Patent Application No. 60/679,834, filed May 11, 2005, whichare incorporated by reference as if fully set forth herein.

FIELD OF INVENTION

The present invention is related to wireless communication systems. Moreparticularly, the present invention is related to reselecting an accesspoint (AP) in a wireless communication system including a plurality ofAPs and a wireless transmit/receive unit (WTRU).

BACKGROUND

A typical wireless local area network (WLAN) comprises one or more basicservice set (BSS). The BSS comprises an AP with at least one WTRUassociated to the AP. Typically, in a system with multiple BSSs, (andthus multiple APs), there can be great differences between the qualityof the radio connection a WTRU experiences based on which AP the WTRUassociates with. Similarly, the radio resources available to a WTRUdepend highly on which AP to which the WTRU is associated with. Thesystem capacity can be optimized by managing which AP each WTRUassociates and reassociates with during the system operation.

The association of the WTRU and the AP is performed at the instant theWTRU joins the WLAN, (i.e., initial association). AP reselection cantake place after the initial association. There are typically tworeasons for the AP reselection. First, the WTRU may have moved such thatthe WTRU can now be better served by another AP. Second, it may bedesirable for the WTRU to be served by another AP because the radioresources used by the currently serving AP are strained, (i.e., thechannel used by the serving AP is congested), while a neighboring AP hasspare radio resources, (i.e., its channel is less congested). It shouldbe understood that the Load Balancing is therefore the management ofappropriate points of attachment for WTRUs while roaming inside the WLANnetwork.

The AP reselection may be driven by either the WTRU or the network. Inthe WTRU-driven AP reselection, each WTRU is responsible for determiningwhat criteria the WTRU would use for AP reselection. Because differentWTRUs may use different policies, and these policies may be gearedtoward maximizing the quality perceived by only one WTRU rather than thequality perceived by the whole system, WTRU-driven AP reselection is notconsidered to perform optimally for load balancing. However, because itis the WTRU that first experiences the quality of the radio connectionfrom the candidate APs, WTRU-driven AP reselection allows for minimalsignaling and high reaction speed which is useful in cases that the WTRUis moving rapidly or that the conditions of the radio channel varyrapidly.

In the network-driven AP reselection, the AP reselection process iscontrolled by the AP. This allows for consistent load balancing policiesacross all WTRUs in the network and theoretically offers the biggestperformance gains from load balancing. However, the network-driven APreselection may require a great amount of signaling overhead and mayimpair the ability of the WTRU to perform quick AP reselection whenneeded.

SUMMARY

A list of candidate BSSs or APs is included in a medium access control(MAC) message received at a WTRU. The list includes an identity of eachcandidate BSS or AP and a preference indicator which indicates an extentthat the candidate BSS or AP is preferred for selection and whether thecandidate BSS or AP may not be considered for selection. A BSS or AP isselected by the WTRU using the list.

A method for use in an access point includes receiving a request for areselection message from a wireless transmit/receive unit (WTRU). Thereselection message is generated in response to the request andincludes: at least one first identifier of one or more candidate basicservice sets (BSSs) that may be considered for reselection; a preferenceindicator associated with each candidate BSS, to indicate an extent towhich the associated candidate BSS may be preferred for selection; anexpiration time indicator associated with each candidate BSS, toindicate a period of time that the associated candidate BSS may beconsidered for reselection; an indication of a measurement to be takenon a candidate BSS and used as a part of a target BSS reselectiondetermination; and a margin by which a value of the measurement of thecandidate BSS must exceed a corresponding value for a currently selectedBSS. The reselection message is transmitted to the WTRU. A reselectionresponse message is received from the WTRU, indicating the target BSSreselection determination of the WTRU.

A method for use in an AP includes receiving a request for a reselectionmessage from a WTRU. The reselection message is generated in response tothe request, and includes: at least one first identifier of one or morecandidate APs that may be considered for reselection; a preferenceindicator associated with each candidate AP, wherein the preferenceindicator indicate an extent to which the associated candidate AP may bepreferred for selection; an expiration time indicator associated witheach candidate AP, wherein the expiration time indicator indicates aperiod of time that the associated candidate AP may be considered forreselection; an indication of a measurement to be taken on a candidateAP and used as a part of a target AP reselection determination; and amargin by which a value of the measurement of the candidate AP mustexceed a corresponding value for a currently selected AP. Thereselection message is transmitted to the WTRU. A reselection responsemessage is received from the WTRU, indicating the target AP reselectiondetermination of the WTRU.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description, given by way of example and to be understood inconjunction with the accompanying drawings wherein:

FIG. 1 shows a wireless communication system in accordance with thepresent invention;

FIG. 2 is a block diagram of a WTRU used in the system of FIG. 1;

FIG. 3 is a block diagram of an AP used in the system of FIG. 1;

FIG. 4 is a flow diagram of a process for reselecting an AP inaccordance with the present invention;

FIG. 5 is a flow diagram of a handover process in accordance with thepresent invention;

FIG. 6 is a simplified block diagram of a WTRU 600 utilizing MACmessaging; and

FIG. 7 is a simplified block diagram of an AP 700 utilizing MACmessaging.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the terminology “WTRU” includes but is not limited to a userequipment (UE), a mobile station, a fixed or mobile subscriber unit, apager, or any other type of device capable of operating in a wirelessenvironment. When referred to hereafter, the terminology “AP” includesbut is not limited to a Node-B, a base station, a site controller or anyother type of interfacing device in a wireless environment.

The features of the present invention may be incorporated into anintegrated circuit (IC) or be configured in a circuit comprising amultitude of interconnecting components.

FIG. 1 shows a wireless communication system 100 in accordance with thepresent invention. The wireless communication system 100 includes aplurality of BSSs 102 a-102 n. Each BSS 102 a-102 n comprises arespective AP 104 a-104 n. An association of a WTRU 106 and an AP, suchas AP 104 a, is performed when the WTRU 106 joins the BSS 102 a coveredby the AP 104 a. Thereafter, AP reselection may take place as the WTRU106 moves away from the currently associated AP 104 a or the radiocondition in the currently associated AP 104 a changes. The coverage ofthe different APs 104 overlap.

In accordance with the present invention, an AP reselection triggeringfunction is located in the WTRU 106 and an AP reselection policy is setby the AP 104 a-104 n. FIG. 2 is a block diagram of the WTRU 106 used inthe wireless communication system 100 of FIG. 1. The WTRU 106 of FIG. 2includes a measurement unit 202 and an AP selection controller 204. Themeasurement unit 202 performs measurements in accordance with the APreselection policy, and the AP selection controller 204 determineswhether the measurement results satisfies a condition defined by the APreselection policy for reselecting a new AP and, if so, the WTRUperforms reselection to the new AP.

FIG. 3 is a block diagram of the APs 104 a-104 n used in the wirelesscommunication system of FIG. 1. The AP 104 of FIG. 3 includes an APreselection policy controller 302 for generating and sending at leastone AP reselection policy and/or a set of state transition triggers tothe WTRU 106, and an optional measurement unit 304 for controlling thestate of the WTRU 106, (i.e., an open state or a closed state).

When the WTRU 106 considers reselecting a new AP, the decision is basedon at least one AP reselection policy generated by the AP 104 a-104 n.In the case where multiple reselection policies are sent from the AP 104to the WTRU 106, the AP 104 preferably signals to the WTRU 106 whichpolicy, (or set of policies), should be used in reselecting a new AP.

FIG. 5 is a flow diagram of a handover process that is implemented in awireless communication system including a WTRU 505, an old AP 510 and anew AP 515 in accordance with the present invention.

In an optional step 520 of FIG. 5, the WTRU 505 requests the old AP 510for an AP-reselection policy, triggers and configuration parameters. Instep 525, the old AP 525 sends an AP-selection policy, triggers andconfiguration parameters to the WTRU 505. In step 530, the WTRU 505evaluates AP-reselection triggers. In step 535, a trigger occurs. Inoptional step 540, the WTRU 505 notifies the old AP 510 of theoccurrence of a trigger condition and selected new AP 515. In step 545,the WTRU 505 initiates a re-association procedure from the old AP 510 tothe new AP 515.

The AP reselection policy is defined by one or a combination of thefollowing information:

1) the measurement(s) (or quantity derived from the measurements) theWTRU has to use as part of the AP-selection procedure;

2) a margin by which the value of a measurement, (or the quantityderived from the measurement) performed on a candidate AP must exceedthe corresponding value for the currently selected AP;

3) a duration for which the margin has to be exceeded before APreselection takes place, (one duration per policy);

4) a list of candidate APs or a list of frequency channels that theWTRUs has to consider in the AP reselection process; or reversely, alist of candidate APs or list of frequency channels that the WTRUs mustnot consider, or a list of APs plus a associated preference values asrecommended by the AP, (i.e., a list of associated side information perAP or group of APs like load or the like).

5) optionally, if more than one policy is specified, an indicationwhether the AP reselection takes place when any, or all, of the policieshave its conditions satisfied;

6) optionally, a validity time for the AP reselection policy, (i.e.expiry time period for the AP reselection policy); and

7) optionally, an indication whether the AP reselection policy applieswhen a WTRU is in an associated state, in an authenticated but notassociated state, or in an unauthenticated and unassociated state.

The measurements to be taken by the WTRU 106 include at least one of,but are not limited to, a load metric indicating the amount of radioresources that are used, received signal quality, (such as asignal-to-noise ratio (SNR), a signal-to-interference and noise ratio(SINR), a received signal power level, or the like), an expectedinstantaneous data rate achievable for transmitting or receiving packetsto/from the AP based on the received signal quality, an expected averagethroughput (EAT) achievable for transmitting or receiving packetsto/from the AP based on channel utilization (CU) and the received signalquality. The CU is defined as the fraction of time the wireless mediumis perceived as “busy” on the frequency channel. The EAT may beestimated by the following formula: EAT=data rate.times.(1-CU).

The load metric may be a channel-based load metric or an AP-based loadmetric. The channel-based load metric includes the fraction of the timeduring which the channel is free on the frequency channel that the APoperates on. The channel-based load metric is equivalent to (1-CU). TheAP-based load metric includes, but is not limited to, allocated timeunits and the number of queued packets.

The margin, (i.e., hysteresis), is defined to avoid prematurelyreselecting a new AP and subsequently reverting to the original APleading to frequent switching back and forth between APs 104 a-104 n.

When a WTRU 106 is already associated to an AP, such as AP 104 a, theWTRU 106 may or may not be considering reselecting another AP 104 b-104n. In accordance with the present invention, two states are defined forthe WTRU 106: a closed state and an open state. In the closed state, theWTRU 106 is not applying any AP reselection policy and does not considerreselecting an AP. In the open state, the WTRU 106 applies the APreselection policy and reselects an AP when the condition stated in theAP reselection policy is met.

In accordance with one embodiment of the present invention, the WTRU 106is always in the open state and, therefore, always applies the APreselection policy. Alternatively, the WTRU 106 switches from the closedstate to the open state, or vice versa, if a certain state transitiontriggering condition is met. The state transition triggering conditionis met if a measurement result exceeds, (above or below depending on thenature of the measurement), a corresponding threshold for apre-specified period of time. Alternatively, the AP 104 a can switch thestate of the WTRU 106, (or group of WTRUs in its BSS), between the openstate and the closed state by explicit signaling if a state transitiontriggering condition as observed by the AP 104 a for the WTRU 106 ismet.

The state transition triggering condition may be monitored by the WTRU106, the AP 104 a-104 n or both. If the WTRU 106 monitors the statetransition triggering condition, the WTRU 106 autonomously switches thestate, once the state transition triggering condition is met. If the AP104 a-104 n monitors the state transition triggering condition, the AP104 a-104 n sends a signal to the WTRU 106 to change the state by meansof layer 2 (L2) or layer 1 (L1) signaling once the state transitiontriggering condition is met.

The state transition triggering condition for switching the state is metif one of the following events, but not limited to, occur. For theconditions “above” and “below” in the following events, the condition inthe parenthesis applies to change the state from the open state to theclosed state, and the condition not in the parenthesis applies to changethe state from the closed state to the open state. Some of thesetriggers could be measured at either the AP or STA side. For example,the data rate of transmitted packets could be monitored by the AP 104(downlink). Based on this, the AP 104 could determine to put the WTRU106 in an open state. When the measurement can be performed from the APside, then this applies for the case where the AP 104 sets the state ofthe WTRU, whereas when the measurement is performed from the WTRU side,then it applies for the case where the WTRU sets its state itself.

The above-mentioned state transition triggering conditions may include:

1) an error rate of transmitted or received packets above (below) athreshold for a predetermined period of time;

2) a medium access delay, (observed either on the contention free accessportion or observed for a particular IEEE 802.11e/wireless multimedia(WMM) access category), above (below) a threshold for a period of time;

3) a data rate of transmitted packets below (above) a threshold for aperiod of time;

4) a data rate of received packets below (above) a threshold for aperiod of time;

5) a contention window size above (below) a threshold for a period oftime;

6) channel occupancy above (below) a threshold for a period of time;

7) a rate of failed packet receptions above (below) a threshold for aperiod of time;

8) a received signal quality, (such as a received signal strengthindicator (RSSI), a received channel power indicator (RCPI), an SNR, anSINR or any other relevant measurement), of the current associationbelow (above) a threshold for a period of time;

9) timeout of a pre-configured or dynamically re-configured associationevaluation timer value;

10) reception of a signaling bit flag, information field or informationelement part of an existing or new medium access control (MAC) frame,preferably of a management type; and

11) any combination of the above.

The thresholds may be relative or absolute values and the time periodmay be defined by absolute values or relative values to the occurrenceof certain events, such as beacon frame transmission.

The AP 104 a-104 n sends unicast or multicast/broadcast-type of MACsignaling frames to the WTRU 106 to send the AP reselection relatedinformation, such as AP reselection policy(ies) and for the statetransition triggering conditions. This information may be sent atregular intervals, when polled, (i.e., when the WTRU 106 explicitlyrequested this information from the AP 104 a-104 n), in an unsolicitedmanner or any combination thereof.

The MAC signaling frame containing AP reselection relevant informationcan be sent to the WTRU 106 in either an associated state, when it is inan authenticated but unassociated state or when it is in anunauthenticated and unassociated state. Different AP reselectionpolicies and state transition triggers may be selected depending on thecurrent status of the WTRU 106. Alternatively, the information may besent to the WTRU 106 as part of an existing MAC frame with or withoutmodification, (management, control or data frame). The AP reselectionrelevant information may be included as an information field orinformation element in either the header or the frame body of the MACframe. For example, the state transition flag for switching between theclosed state and the open state may be sent by the AP 104 a-104 n usingone of the currently unused and reserved bits or combination values inany part of the existing IEEE 802.11 MAC preamble, header or frame bodyof the MAC frames. Although most of the examples provided above relatedto L2 frames, one of ordinary skill in the art may implement the presentinvention using any other type of frames to support higher layerprotocol exchanges between a WTRU and any network entity, such as thoseused to simple manage network protocol (SNMP) via an AP, managementinformation bases (MIBs), or the like. Rather that explicitly signalingframes, the same can be achieved implicitly by protocol design. Forexample, an LB frame specified as being part of a class 3 frame may onlyapply to associated and authenticated states.

FIG. 4 is a flow diagram of a process 400 for reselecting an AP inaccordance with the present invention. A first AP currently associatedwith a WTRU 106, generates and sends at least one AP reselection policyand/or a set of state transition triggers to the WTRU 106 (step 402). Instep 404, the WTRU determines whether it is in a closed state or an openstate. If the WTRU is determined to be in an open state at step 404, theWTRU 106 then performs measurements in accordance with the APreselection policy currently in force (step 406). The AP reselectionpolicy defines required measurements and related parameters as statedabove. The WTRU 106 then determines whether the measurement resultssatisfy a condition defined by the AP reselection policy (step 408). Thecondition is satisfied if the measurement results, (or a value derivedfrom the measurement results), for a new AP exceeds, (above or belowdepending on the nature of the measurement), a corresponding thresholdby the margin for the duration defined in the AP reselection policy. Ifthe condition is satisfied, the WTRU 106 performs reselection to asecond AP (step 410). If not, the currently selected AP is maintainedand the process 400 returns to step 404.

In case that more than one candidate AP meets the condition defined bythe policy, the WTRU 106 may select an AP whose measurement valueexceeds that of the currently selected AP by the largest amount.

The AP 104 a-104 n may define multiple policies and the WTRU 106 mayoptionally use a combination of policies to determine which AP toreselect to. In this case, the WTRU 106 may elect to reselect a new APif any one of the policies in the combination has its conditionssatisfied, or alternatively if all policies in the combination havetheir conditions satisfied. Preferably, the AP 104 a-104 n signals tothe WTRU 106 which policy, (or set of policies), should be used inreselecting a new AP.

FIG. 6 is a simplified block diagram of a WTRU 600 utilizing MACmessaging. A MAC 601 has a MAC message generator 602 which produces MACmessages and a MAC message receiver 603 which receives MAC messages. Atransceiver 604 transmits the produced MAC messages and receives MACmessages utilizing an antenna or antenna array 605.

FIG. 7 is a simplified block diagram of an AP 700 utilizing MACmessaging. A MAC 701 has a MAC message generator 702 which produces MACmessages and a MAC message receiver 703 which receives MAC messages. Atransceiver 704 transmits the produced MAC messages and receives MACmessages using an antenna or antenna array 705.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention.

1. A method for use in an access point, comprising: receiving a requestfor a reselection message from a wireless transmit/receive unit (WTRU);generating the reselection message in response to the request, thereselection message including: at least one first identifier of one ormore candidate basic service sets (BSSs) that may be considered forreselection; a preference indicator associated with each candidate BSS,wherein the preference indicator indicates an extent to which theassociated candidate BSS may be preferred for selection; an expirationtime indicator associated with each candidate BSS, wherein theexpiration time indicator indicates a period of time that the associatedcandidate BSS may be considered for reselection; an indication of ameasurement to be taken on a candidate BSS and used as a part of atarget BSS reselection determination; and a margin by which a value ofthe measurement of the candidate BSS must exceed a corresponding valuefor a currently selected BSS; transmitting the reselection message tothe WTRU; and receiving a reselection response message from the WTRUindicating the target BSS reselection determination of the WTRU.
 2. Themethod according to claim 1, wherein the reselection message indicates afrequency channel of each candidate BSS.
 3. The method according toclaim 1, wherein the reselection message indicates load information foreach candidate BSS.
 4. The method according to claim 1, wherein thereselection message is included in a medium access control frame.
 5. Themethod according to claim 1, wherein the reselection message furtherincludes at least one second identifier of one or more candidate BSSsthat may not be considered for reselection.
 6. A method for use in anaccess point (AP), comprising: receiving a request for a reselectionmessage from a wireless transmit/receive unit (WTRU); generating thereselection message in response to the request, the reselection messageincluding: at least one first identifier of one or more candidate APsthat may be considered for reselection; a preference indicatorassociated with each candidate AP, wherein the preference indicatorindicate an extent to which the associated candidate AP may be preferredfor selection; an expiration time indicator associated with eachcandidate AP, wherein the expiration time indicator indicates a periodof time that the associated candidate AP may be considered forreselection; an indication of a measurement to be taken on a candidateAP and used as a part of a target AP reselection determination; and amargin by which a value of the measurement of the candidate AP mustexceed a corresponding value for a currently selected AP; transmittingthe reselection message to the WTRU; and receiving a reselectionresponse message from the WTRU indicating the target AP reselectiondetermination of the WTRU.
 7. The method according to claim 6, whereinthe reselection message indicates a frequency channel of each candidateAP.
 8. The method according to claim 6, wherein the reselection messageindicates load information for each candidate AP.
 9. The methodaccording to claim 6, wherein the reselection message is included in amedium access control frame.
 10. The method according to claim 6,wherein the reselection message further includes at least one secondidentifier of one or more candidate APs that may not be considered forreselection.